It is often desirable to place antennas near and parallel to metallic surfaces. However these surfaces reflect electromagnetic waves out of phase with the incident wave, thus short circuiting the antennas. While naturally occurring materials reflect electromagnetic waves out of phase, artificial magnetic conductors (AMCs) are metasurfaces that reflect incident electromagnetic waves in phase. An Artificial Magnetic Conductor (AMC) is a type of metamaterial that emulates a magnetic conductor over a limited bandwidth. See, in this regard, Gregoire, D.; White, C.; Colburn, J.; “Wideband artificial magnetic conductors loaded with non-Foster negative inductors,” Antennas and Wireless Propagation Letters, IEEE, vol. 10, 1586-1589, 2011 (hereinafter Gregoire) and D. Sievenpiper, L. Zhang, R. Broas, N. Alexopolous, and E. Yablonovitch, “High-impedance electromagnetic surfaces with a forbidden frequency band,” IEEE Trans. Microw. Theory Tech., vol. 47, pp. 2059-2074, November 1999 (hereinafter Sievenpiper).
An AMC ground plane enables conformal antennas with currents flowing parallel to the surface because parallel image currents in the AMC ground plane enhance their sources. In the prior art, AMCs have been realized with a laminated structure composed of a periodic grid of metallic patches distributed on a grounded dielectric layer. See, in this regard, the prior art documents mentioned above as well as: F. Costa, S. Genovesi, and A. Monorchio, “On the bandwidth of high-impedance frequency selective surfaces”, IEEE AWPL, vol. 8, pp. 1341-1344, 2009 (hereinafter Costa).
AMCs are typically composed of unit cells that are less than a half-wavelength in size and achieve their properties by resonance. But such AMCs have limited bandwidth. Their bandwidth is proportional to the substrate thickness and its permeability. See, in this regard, the prior art documents mentioned above as well as: D. J. Kern, D. H. Werner and M. H. Wilhelm, “Active negative impedance loaded EBG structures for the realization of ultra-wideband Artificial Magnetic Conductors,” Proc. IEEE Ant. Prop. Int. Symp., vol. 2, 2003, pp. 427-430 (hereinafter Kern). At VHF-UHF frequencies, the thickness and/or permeability necessary for reasonable AMC bandwidth is excessively large for antenna ground-plane applications.
A passive AMC typically comprises metallic patches disposed above a ground plane with via holes connecting the patches to the RF ground with a dielectric medium between the patches and the RF ground. Passive AMCs must be very thick to have the operational bandwidths comparable to those achievable with much thinner active AMCs (AAMCs).
AAMC technology is applicable to a number of antenna applications including:
(1) increasing antenna bandwidth (see in this regard: White, C. R.; May, J. W.; Colburn, J. S.; “A variable negative-inductance integrated circuit at UHF frequencies,” Microwave and Wireless Components Letters, IEEE, vol. 21, no. 12, pp. 35-37, 2011 (hereinafter White) and S. E. Sussman-Fort and R. M. Rudish, “Non-Foster impedance matching of electrically-small antennas,” IEEE Trans. Antennas Propagat., vol. 57, no. 8, August 2009 (hereinafter Sussman-Fort).
(2) reducing finite ground plane edge effects for antennas mounted on structures to improve their radiation pattern,
(3) reducing coupling between closely spaced (<1λ) antenna elements on structures to mitigate co-site interference,
(4) enabling the radiation of energy polarized parallel to and directed along structural metal surfaces, and
(5) increasing the bandwidth and efficiency of cavity-backed slot antennas while reducing cavity size.
This AAMC technology is particularly applicable for frequencies <1 GHz where the physical size of the traditional AMC become prohibitive for most practical applications.
Active circuits (e.g. negative inductors or NFCs) may be employed to increase the bandwidth of a AMC, thus constituting the AAMC. The AAMC is loaded with non-Foster circuit (NFC) negative inductors to increase it bandwidth by 10 times or more. When the AMC is loaded with the NFC, its negative inductance in parallel with the substrate inductance results in a much larger net inductance and hence, a much larger AMC bandwidth. An AAMC architecture is shown in FIG. 1. However, the mere inclusion of NFCs means that the AAMC is conditionally stable and the NFCs must be designed properly to avoid undesirable oscillation. In U.S. Pat. No. 8,976,077 issued Mar. 10, 2015 and entitled “Wideband Tunable Impedance Surfaces”, a method of making an AAMC is described using Non-Foster Circuits (NFCs), but it does not disclose how to ensure stability of the AAMC itself which is due to the fact that it was discovered later that NFCs (which were designed with stability in mind) used in the AAMC resulted in instability due to cross coupling in the E-plane. This instability is manifested as an uncontrolled oscillation of the NFCs and spurious emitted radiation. This instability problem is addressed by the present invention.